tb2kmail

tb2kmail is a tool to export mailboxes from The Bat! to a KMail compatible directory structure. It makes use of The Bat!'s export command and thus works under Microsoft Windows only. Scott Haug sent me a patch which makes it possible to create a directory structure which Thunderbird likes.

Prerequisites

I don't think you need an installed copy of the Mailer, it should work with a plain thebat.exe (plus some DLLs maybe), too. However, I haven't tested this.
If you don't have it yet (what do you want to export then?), you can get it here. Think about registering it if you stick with Windows. It's really worth it.

Perl

Maybe you should give ActiveState's Windows port a try. If you use another one, then make shure you've got the following modules (those are all shipped with ActiveState's Perl):